The principal object of the present invention is to provide a fiber ceramic product which can be utilized at high temperatures and is less costly to manufacture than other fiber ceramic products capable of use at such temperatures.
It is a further object of the invention to provide a fiber ceramic product manufactured with low cost alumina-silica ceramic fibers which has the dimensional stability of ceramic fiber products manufactured with high alumina content alumina-silica fibers, and it is a particular object of the present invention to provide refractory shapes and insulating blocks manufactured in this manner.
Refractory insulating blocks containing ceramic fibers are in commercial use in both electric furnaces and other types of furnaces U.S. Pat. No. 3,500,444 of Mar. 10, 1970, issued to W. K. Hesse et al entitled ELECTRICAL HEATING UNIT WITH AN INSULATING REFRACTORY SUPPORT is an example of such a block supporting an electrical heating element. The fibers are more fully described in an article entitled "Critical Evaluation of the Inorganic Fibers" in Product Engineering, Aug. 3, 1964, pages 96 through 100. The fibers used in refractory insulating blocks at the present time are generally of Al.sub.2 O.sub.3 -SiO.sub.2 compositions.
Hesse discloses a process for manufacturing thermal insulating blocks of inorganic fibers in which the fibers are mixed with a body of water to form a random distribution and the water is removed by vacuum through a filter plate. Another example cf a refractory thermal insulating fiber block formed by a vacuum process is that disclosed in the U.S. Pat. No. 4,575,619 of Ludwig Porzky entitled ELECTRICAL HEATING UNIT WITH SERPENTINE HEATING ELEMENT. Fiber blocks produced by suspending inorganic ceramic fibers in a body of water to form a slurry have also been more recently produced by use of vibration to facilitate removal of the liquid component of the slurry as disclosed by U.S. patent application Ser. No. 06/868,651 of Duane L. Sterwald filed May 30, 1986 entitled METHOD OF MAKING THERMAL INSULATING BLOCKS AND ELECTRICAL HEATING UNITS AND THE PRODUCTS THEREOF, now U.S. Pat. No. 4,719,336 issued Jan. 12, 1988.
The present inventors have also discovered that ceramic products may be produced from inorganic fibers by mixing the fibers with a relatively smaller quantity of water, thereafter placing the fibers in a mold, and thereafter absorbing moisture from the mixture in the mold to produce a hardened molded body, as disclosed in their U.S. patent application Ser. No. 06/878,068, filed Jun. 24, 1986, now U.S. Pat. No. 4,935,178 entitled REFRACTORY FIBER PRODUCTS AND METHOD OF MAKING SUCH PRODUCTS.
Alumina-silica fibers have also been added to ceramic composites to improve the mechanical properties of porous ceramics, as described by D. A. Sheldon and David Lewis in an article entitled "Fabrication and Properties of a Ceramic Fiber-Ceramic Matrix Composite", Journal of the American Ceramic Society, August 1976, pages 372-374, and by John R. Baer and David Lewis III in an article entitled "In Vitro Degredation of Ceramic-Ceramic Composite" in Ceramic Bulletin, Vol 57, No. 2 (1978), pages 220-222. The described products are not for high temperature use, and have higher densities than desired for refractory thermal blocks.
R. Ganz and W. Kronert, in an article entitled "Crystallisation Behaviour of High Temperature Ceramic Fibers of the Al.sub.2 O.sub.2 -SiO.sub.2 System", Ceramic Bulletin, Vol. 57 No. 2 (1978), have described the behavior of alumina-silica fibers with increases in temperature up to and including the temperatures experienced by refractory materials. The article concludes that the fibers are substantially amorphous and recrystallize with increased temperature. Experiments with fibers containing about 46% Al.sub.2 O.sub.3 and 53% SiO.sub.2 indicate that up to approximately 1300.degree. C., the recrystallization is in the form of mullite which is relatively stable and forms at the areas of contact of the fibers. Above about 1300.degree. C. recrystallization occurs in the form of cristobalite. The formation of mullite is not accompanied by a marked change in the ratio of the diameter to the length of the fibers, but recrystallization in the form of cristobalite is accompanied by such a change, thus resulting in embrittlement and deterioration of the fiber structure. The Ganz and Kronert article also classifies fibers for service at 1260.degree. C., 1400.degree. C. and 1600.degree. C., those fibers intended for higher temperature use containing a larger ratio of Al.sub.2 O.sub.3 molecules to SiO.sub.2 molecules. Fibers containing a larger ratio of alumina to silica are shown to be more stable dimensionally than fibers containing a lower ratio of alumina to silica.
Silica-alumina fibers for use at higher temperature, such as 1600.degree. C. fibers, must contain a higher percentage of alumina than fibers classified for lower temperature use and are more costly to produce. The Ganz and Kronert article describes methods for producing such fibers with alumina contents up to approximately 65% and states that higher alumina contents cannot be obtained because of the decreased viscosity of the melted ceramic materials. Alumina-silica fibers with greater than 66% alumina are now commercially available at a premium price including fibers with alumina contents as high as 95%.